1. Field of the Invention
The present invention relates generally to a near-field scanning Raman spectroscope and more particularly to apertureless near-field scanning Raman spectroscopy of the reflection type.
2. Description of Related Art
Raman spectroscopy is a technique that has been used for many years to measure molecular vibrations, which can then be used to determine the structure and chemical bonding of a sample. Light which shines on a sample experiences a wavelength shift due to the vibration of the molecules. Information can be determined from the spectrum of wavelengths that occur as to the make-up of the sample and to determine the chemical bonding and information of the constituent atoms. While this well-known system has been utilized to great advantage for decades, it has a spatial resolution in the micron range. It is desirable to extend the range in which this measurement can be used for purposes of examining nano-devices, quantum dots and single molecules of biological samples.
Recently, Raman spectroscopy has been combined with near-field scanning optical microscopy to form a technique known as near-field scanning Raman microscopy (NSRM). This is based on the principle used in near-field scanning optical microscopy (NSOM) of using an optical fiber having a small aperture to deliver laser light and keeping the fiber at a close constant distance (on the order of tens of nanometers) above the sample surface. By using such an optical fiber, it is possible to limit the laser spot size and scan the fiber tip across the sample surface. The resultant Raman signal is collected using a microscopic objective or a lens. The collected light is coupled to a Raman spectrometer where the Raman spectra are recorded for each point on the sample. An NSRM image is constructed using the spectra.
This type of device has been described in a series of published articles including S. Webster, D. A. Smith and D. N. Batchelder, Raman microscopy using a scanning near-field optical probe, Vibrat. Spectrosc. 18 (1998) 51, E. J. Ayars and H. D. Hallen, Surface enhancement in near-field Raman spectroscopy, Appl. Phys. Lett. 76 (2000) 3911, C. L. Jahncke, M. A. Paesler and H. D. Hallen, Raman imaging with Near-field scanning optical microscopy, Appl. Phys. Lett. 67 (1995) 2483, M. N. Islam, X. K. Zhao, A. A. Said, S. S. Mickel and C. F. Vail, High-efficiency and high-resolution fiber-optic probes for near-field imaging and spectroscopy, Appl. Phys. Lett. 71 (1997) 2886, T. Yatsui, M. Kourogi and M. Ohtsu, Appl. Increasing throughput of a near-field optical fiber probe over 1000 times by the use of a triple-tapered structure, Phys. Lett. 73 (1998) 2090, and Y. H. Chuang, K. G. Sun, C. J. Wang, J. Y. Huang and C. L. Pan, A simple chemical etching technique for reproducible fabrication of robust scanning near-field fiber probes, Rev. Sci. Instrum. 69 (1998) 437.
Because this type of system provides the incoming laser light through a very small aperture in the optical fiber, the light is extremely weak. Despite extensive efforts to fabricate fiber tips with a higher throughput, the strength of the light is still quite low. In addition, the Raman signal is intrinsically weak (typically less than 1 in 107 photons). A typical Raman spectrum takes several minutes to collect using this type of near-field technique, making it prohibitive to construct a Raman image this way. For example, Raman images reported in some of the references cited above, took more than 9 hours to collect data.
An alternative approach to the fiber tip is the use of an apertureless metal tip. A system of this type has been discussed in R. M. Stxc3x6ckle, Y. D. Suh, V. Deckert and R. Zenobi, Nanoscale chemical analysis by tip-enhanced Raman spectroscopy, Chem. Phys. Lett 318 (2000) 131. In this system, the light is not provided from the fiber tip, but instead a metal tip is placed within a focused light beam in order to enhance the Raman signal by several orders of magnitude. It is believed that this enhancement is a result of local field enhancement by the metal tip which is either due to the enhancement of the electromagnetic field in the vicinity of sharply pointed metal needle, or due to the excitation of surface plasma by the laser electromagnetic wave, or increased polarizability of the sample due to the interaction between the tip and the sample. This type of system has been used to record Raman images using a transmission mode. That is, the sample must be either transparent or very thin so that the light can travel therethrough. While this type of technique is a major step forward, it has the drawback that many samples are not transparent, and that it is often desirable not to take thin samples. Accordingly, this type of technique is not suitable for a reflection type geometry which is desirable in many cases.
Accordingly, one object of the present invention is to provide an apertureless near-field scanning Raman microscopy system.
Another object of the present invention is to provide a novel near-field scanning Raman microscopy system using reflection geometry.
A still further object of the present invention is to provide an apertureless near-field scanning Raman microscopy system using reflection geometry.
A still further object of the present invention is to provide a nano-scopic Raman imaging technique using near-field enhancement.
Another object of the present invention is to provide an apertureless near-field scanning Raman microscopy system using a bent metal tip to enhance the near-field signal.
Briefly these and other objects of the present invention are achieved by providing a laser beam focused by a microscopic objective to a small spot on a sample with a bent metal tip touching (contacting with or close to) the surface within the spot of light. The presence of the metal tip enhances the near-field signal so that the quality of the Raman spectrum is improved and so that that time necessary to collect the spectrum is much smaller.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.